Extended Rodlike Polyaromatic Receptors with Bent Tridentate Units Complexed to Lanthanide Metal Ions

A synthetic strategy is developed to attach semirigid lipophilic sidearms to the 6-positions of bent aromatic tridentate 2,6-bis(benzimidazol-2-yl)pyridine cores to produce U-shaped ligands, L^6,7. Differential scanning calorimetry (DSC) reveals that entropic contributions severely affect the isotropization processes of these flexible receptors, but no mesomorphism is detected. The attachment of oxygen linkers to the 5- or 6-positions of the benzimidazole sidearms lowers the ligand-centered ¹ππ* and ³ππ* excited states, and the semiempirical ZINDO method assigns this effect to a destabilization of the HOMO orbitals resulting from π-interactions. Reactions of L⁶ with Ln(NO₃)₃·xH₂O provide the rodlike 1:1 complexes [Ln(L⁶)(NO₃)₃] (Ln = La−Lu), which are stable in the solid state but partially dissociate in acetonitrile. The crystal structure of [Lu(L⁶)(NO₃)₃]·CH₃CN (18a, LuC₆₃H₈₄N₉O₁₃, monoclinic, P2₁/n, Z = 4) reveals an I-shaped arrangement of the ligand strand arising from the meridional complexation of the bent tridentate unit to nine-coordinate Lu(III). The replacement of nitrate anions with trifluoroacetate anions gives the centrosymmetric dimer [Lu(L⁶)(CF₃CO₂)₃]₂ (23, Lu₂C₁₃₄H₁₆₂N₁₀O₂₀F₁₈, triclinic, P1̄, Z = 1), in which the symmetry-related Lu atoms are connected by two bridging carboxylates, leading to an H-shaped dimetallic edifice. These complexes [Ln(L⁶)(NO₃)₃] and [Ln(L⁶)(CF₃CO₂)₃]₂ fulfill the geometrical criteria required by precursors of calamitic metallomesogens, but no mesomorphism can be detected, while photophysical studies indicate that the low energies of ligand-centered ³ππ* excited states drastically limit the luminescence of Eu(III) complexes. The relationships between structural and electronic properties resulting from 5- or 6-substitutions of the benzimidazole rings and the effects of these substitutions on photophysical and thermal properties are discussed.